Analysis of cold bending cracking of martensitic stainless steel castings

Metal bending test is a test method to measure the bending plastic deformation ability of metal materials under bending load, which is one of the basic methods of material mechanical properties test. The bending test is mainly used to measure the bending strength of brittle and low plastic materials (such as cast iron, high carbon steel, tool steel, etc.) and to reflect the deflection of plastic index. For brittle materials, the bending test can only produce a small amount of plastic deformation. For plastic materials, bending fracture strength can not be measured, but ductility and uniformity can be tested. The bending test of plastic material is usually called cold bending test. In the test, the sample is loaded to bend to a certain extent, and the surface of the sample is observed for cracks. But for high plastic materials, bending test usually can not reach the damage degree, so bending strength test is generally not done.

Bending test can also be used to check the surface quality of materials. In the bending test, the stress distribution on the sample section is uneven, the surface stress and strain is the largest, and the core is the smallest, so it can more sensitively reflect the surface casting defects of materials, so it is often used to detect the surface quality of materials, compare and identify the quality and performance of carburized layer and surface quenching layer after surface heat treatment. It can also test the ability of plastic deformation and casting defect display of material or welded joint.

Figure shows the finite element ABAQUS software used to simulate the deformation and stress of the material during cold bending. According to the above characteristics of the bending test and the software simulation results, it can be seen that the stress distribution on the section of the cold-formed sample is uneven, the surface stress and strain is the largest, and the core is the smallest. In particular, the outer surface of the bending part of the bending specimen is subjected to great tensile stress. At this time, if there are many non-metallic inclusions in the sample, and the size of the inclusions is large and with edges and corners, it is introduced that the inclusions play a role in splitting the metal matrix, destroying the continuity of the metal matrix, producing stress concentration around the inclusions, making the metal produce micro pores, and gradually extend and expand as the external force increases, becoming the crack source, The metal matrix is separated from each other, resulting in cracks. In addition, the plastic toughness and brittleness of inclusions are poor, and the ability to resist deformation is different from that of base metal, which leads to cracks at inclusions and makes the bending sample unqualified.

In addition, if the casting matrix contains δ- Ferrite, an abnormal structure, can also reduce the bending properties. Because δ- Ferrite belongs to the structure formed in the high temperature stage of casting solidification, and belongs to softening phase, δ- The hardness of ferrite is about hv10165, while that of martensite is about hv10240 after twice tempering. When the outer surface of the specimen is subjected to a large tensile stress, if there is a tensile stress in the matrix of the specimen δ- When the hardness and other properties of ferrite are inconsistent, the overall plastic deformation resistance of ferrite will be reduced, and it will be preferentially increased δ- The ferrite phase region produces crack source and propagates, which makes the bending sample unqualified.

For the casting defects of the sample itself, such as micro pores or shrinkage porosity, slag holes, which can destroy the continuity and compactness of the metal, whether it is leakage or in the subsurface layer, especially in the bending outside of the bending sample, it will greatly reduce the ability of the sample to resist bending deformation, and there is a large local stress concentration in the casting defects, And it becomes the congenital crack source, so it is easy to make the bending unqualified.

It can be seen that if there are one or more casting defects on the bending sample of casting, it is easy to reduce the bending performance and increase the risk of unqualified bending test. Of course, there are other factors that lead to unqualified bending, such as irregular machining sample preparation, improper size, large processing stress, poor surface roughness or tool marks, oxidation decarburization, carburization, metallization, coarse grains, mixed grains, uneven structure of the original surface of the sample, which may also lead to poor bending performance and unqualified bending.